Hot melt adhesives are known as useful materials. At room temperature, they are solid or have very high viscosity, but at elevated temperatures they have much lower viscosity and can be conveniently applied to a substrate. A second substrate can be contacted with the melted adhesive layer, and the adhesive bond becomes stronger as the composition cools and the adhesive re-solidifies or re-builds high viscosity. Because the cooling happens relatively quickly, the composition or composite article develops good green strength (i.e., the bond is reasonably strong soon after the materials are brought together). Hot melt adhesives are often based on thermoplastic polymers, and composite articles made with such adhesives often develop good green strength, but they generally lose strength when the article is heated or when it is exposed to solvents.
In contrast, curable adhesives (synonymous herein with “reactive adhesives”), once cured, generally maintain their adhesive properties when heated and/or exposed to solvents. However, such adhesives generally are fluid before they cure and thus have poor green strength.
A class of materials that has a useful level of green strength and has the performance advantages of curable adhesives is the reactive hot melt (RHM) adhesive. These are materials that are solid or have very high viscosity at room temperature, but they have low viscosity at elevated temperatures and can be used to assemble composite articles in a manner similar to that employed with thermoplastic hot melt adhesives. Then, as the RHM cools, it develops some green strength quickly. Later, as its curing reaction proceeds over time, the RHM adhesive develops the full advantages of a cured reactive adhesive. RHM compositions based on a mixture of an isocyanate-functional prepolymer based on a polyether polyol, a polyester polyol, or mixtures thereof, and a polyether polyester thermoplastic elastomer are disclosed by Anderson et. al. in U.S. Pat. No. 5,939,499. The isocyanate groups on isocyanate-functional compositions are believed to form bonds with each other by a reaction with water, and such compositions are examples of moisture-curable (also called “moisture reactive”) compositions.
The use of cristobalite silica as a filler in various polymers has been discussed by R. Halvorsen in “Cristobalite: A Unique Form of Silica” in Adhesives Age, August 1997, pp. 58-62.
RHM adhesives generally have better green strength than ordinary curable adhesives, but further improvements in green strength are desirable, to improve the ease of handling bonded articles as soon as possible after assembly. I have found that the inclusion of cristobalite silica in moisture-curable hot-melt adhesive compositions improves the green strength of those compositions.